It is beneficial to equip opto-electronic circuits with a means to adjust the signal intensity within the circuit and/or at the output of the circuit. A power-adjust capability is useful to compensate either for unwanted optical losses within the circuit, to compensate for the ageing of the optical source(s) within or external to the circuit, and/or to adjust the output power level of the circuit.
One method to enable adjustments to the strength of an optical signal within or at the output of an optical circuit, is to dispose a semiconductor optical amplifier (SOA) somewhere along a light path within or at the output of the optical circuit. This approach can be used in optical circuits that comprise an assembly of discrete components, a hybrid assembly combining discrete and monolithically integrated components, or a fully monolithically integrated optical circuit.
The present invention relates to the particular application of the amplification of light prior to the input of the light into a Mach-Zehnder (MZ) optical modulator or switch, and more specifically to the case where an SOA is monolithically integrated with a Mach-Zehnder modulator. Previous workers have placed a SOA either along the access waveguide leading to the input of the modulator, or have placed one SOA in each arm of a MZ modulator or switch.
The SOA's thus configured can provide the general benefits to the optical circuit of optical power adjustment, as described earlier. In the particular configuration where a SOA is placed in each arm of a MZ modulator, it is then possible to adjust the gain of each SOA and thereby adjust the ratio of the power in the two arms of the MZ interferometer. It has been shown that an unequal division of input power in the arms of the MZ can be beneficial to transmission performance over fibre.
However the placement of a SOA before a Mach-Zehnder, or the placement of a SOA in each arm of a Mach-Zehnder, introduces some drawbacks.
First, the addition of a SOA either before a Mach Zehnder, or the addition of a SOA in each arm of a Mach-Zehnder, increases the size of the optical circuit.
Secondly, the placement of a SOA before a MZ can degrade the performance of the circuit, because spontaneous emission noise that is generated within the SOA can reach other parts of the optical circuit, particularly in the case where the SOA and MZ are integrated with a laser for example.
The placement of one SOA in each arm of a Mach-Zehnder interferometer enables the adjustment of the ratio of the power in the two arms. However if the bias voltage that must be supplied to the two SOAs is unequal, then the requirement for a circuit that divides the bias current between the two SOAs unequally, or the requirement for an additional current source so that each SOA can be biased independently, introduces extra assembly and parts cost, and may also increase the overall size and power dissipation of the circuit.
The invention disclosed here either eliminates or reduces the severity of all the drawbacks described here.